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"Optical frequency combs have become pivotal tools in a wide range of studies, from metrology, gas sensing to astronomy. In 2007 the host laboratory has demonstrated an entirely new approach to generate frequency combs from optical micro-resonators. The physical process responsible for the comb generation is parametric four-wave mixing.The micro-resonator frequency combs not only represent considerably reduction in size, complexity and power consumption, but moreover, the small circumference of optical micro-resonators enable for the first time to generate frequency combs with repetition in the technologically and scientifically interesting range of above tens of gigahertz. The high repetition rates have potentials in applications such as high capacity telecommunications and astronomical spectrometercalibration.The main experimental goal of this proposal is to generate frequency combs in the scientifically and technologically significant ""molecular fingerprint"" mid-infrared spectral region in micro-resonators made of crystalline materials. The crystalline materials feature exceptionally low loss from UV to mid-infrared, and resonators with cavity finesse up to 1 million have been successfully fabricated in the host group. It is envisioned that broadband combs in the mid-IR can be generated in such micro-resonators, and such a mid-IR comb will greatly facilitate molecular spectroscopy, enables multi-heterodyne spectroscopy in the mid-IR and will also have potential in astronomical spectrometer calibration.

The applicant is a female scientist who has obtained her undergraduate degree in Taiwan and her PhD at Harvard University (USA) in the group of Quantum Cascade Lasers pioneer Federico Capasso. She started her postdoctoral work in September 2009 at the Max-Planck Institute for Quantum Optics in the host group in the division of Prof. Theodor Haensch. Her experience makes her the ideal candidate to carry out the proposed research project."

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A comb to untangle the molecular structure of materials

Scientists have demonstrated for the first time a novel type of spectroscopy in the frequency region relevant to resolution of molecules. The compact device size will make it easily adaptable by research and hospital labs around the world.

A graphical representation of an optical frequency comb can be described as having very fine teeth in a beautiful continuum of all possible colours and different lengths. Its spectrum is produced from millions of ultra-short, closely spaced pulses of laser light of different colours, corresponding to the millions of different frequencies (or wavelengths) in the electromagnetic spectrum.
Frequency combs, introduced in the 1990s, have revolutionised measurements of frequency and time. They are widely used in optical metrology, have paved the way to atomic clocks and have enabled the highest resolution yet in laser spectroscopy.
Entering the realm of molecular spectroscopy, their enormous spectral coverage together with the very high spectral resolution of each 'tooth' of the comb enables the identification of individual atoms and molecules. However, until now, spectroscopy in the mid-infrared (IR) range corresponding to wavelengths of 2 to 20 micrometres, the so-called 'molecular fingerprint', was used minimally by medical and scientific specialists. This was due to the bulky nature of the systems imposed by an indirect method of producing the mid-IR spectrum.
Scientists working on the EU-funded project 'Monolithic frequency comb generators in the mid-infrared' (IRCOMB) have demonstrated for the first time the direct production of mid-IR frequency combs based on novel techniques and choice of materials that enable compact packaging. The miniaturised source consists of a crystalline microresonator pumped with a continuous-wave laser. Using a non-linear process called four-wave mixing, scientists have produced a broad comb spectrum near 2.5 micrometres that is poised to revolutionise science and medicine.
IRCOMB has extended frequency comb spectroscopy into the molecular fingerprint region in a small, compact device suitable for use by medical and scientific experts. The extremely high-resolution technology promises to open new windows of understanding of the molecules that make up engineered materials, living organisms and the Universe.

Optical frequency combs, that is, broad spectral bandwidth coherent light sources consisting of equally spaced sharp lines, revolutionised optical frequency metrology one decade ago. They now enable dramatically improved acquisition rates, resolution and sensitivity for molecular spectroscopy mostly in the visible and near-infrared ranges. The mid-infrared spectral range (approximately 2 - 20 µm) is known as the 'molecular fingerprint' region as many molecules have their characteristic, fundamental vibrational bands in this part of the electromagnetic spectrum. Broadband mid-infrared spectroscopy therefore constitutes a powerful and ubiquitous tool for optical analysis of chemical components that is used in biochemistry, astronomy, pharmaceutical monitoring and material science. Mid-infrared frequency combs have therefore become highly desirable. The advent of a compact and versatile coherent light source in this region came after the invention of quantum cascade lasers (QCLs) in 1994. However, due to its intrinsic ultrafast gain recovery time, QCLs are very difficult to be mode-locked, which impedes its potential to become an optical frequency comb source in the mid-infrared. Today, the most common approach to create frequency combs in the mid-infrared is to frequency down-convert a near-infrared comb through nonlinear processes, such as optical parametric oscillation or difference frequency generation.

Under the project IRCOMB carried out in the laboratory of Tobias Kippenberg between October 2011 and March 2013, we demonstrate a novel, direct route to mid-infrared frequency comb generation based on ultra-high Q (quality factor) crystalline optical microresonators. The underlying mechanism is cascaded four-wave mixing caused by the third-order Kerr nonlinearity in high-Q whispering-gallery mode (WGM) microresonators, which was first demonstrated in silica microtoroids in the near infrared. Two pump photons are annihilated to give rise to a pair of photons with frequency shifted up and down. This process can cascade and thereby lead to the formation of a so-called 'Kerr comb'. Several microresonator platforms based on this mechanism have demonstrated Kerr frequency comb generation in the near-infrared region, such as silica microtoroids, silicon nitride, Hydex glass, crystalline fluorides and fused quartz. Most of the platforms, however, principally cannot operate in the midinfrared region. In the project, by careful choice of resonator material and design, we demonstrate for the first time mid-infrared Kerr frequency combs based on microresonators. By pumping an ultra-high Q crystalline microresonator made of magnesium fluoride (MgF2), we observed the generation of a broadband Kerr comb at 2.5 mm spanning 200 nm (approximately 10 THz) with a line spacing of 100 GHz (Nature Communications, 2013). The particular advantages of microresonator-based frequency combs are the compact form factor, large comb mode spacing and high power per comb line. In addition, we have carefully characterised their phase noise and shown that the mid-infrared Kerr combs indeed consist of narrow and mutually coherent lines. The current approach is suitable for extending comb generation further into the mid-infrared.

In addition to the main objective of creating frequency comb in the mid-infrared from microresonators, there are other results obtained during the post-doctorate project:

Understanding the formation dynamics and noise in microresonator based frequency combs

Over the past years significant advances in Kerr comb technology has been achieved. However, one important aspect - phase noise - has not been understood, which is usually observed experimentally in the form of linewidth broadening and multiple repetition-rate beat notes. Understanding the underlying processes responsible for phase noise is not only an outstanding scientific challenge, but indeed crucial for bringing the Kerr comb technology to maturity. We have undergone a thorough and systematic study both experimentally and theoretically of phase noise phenomena in crystalline MgF2 and Si3N4 planar microresonators, and revealed the universal, platform-independent dynamics of Kerr comb formation (Nature Photonics, 2012). This allows both the explanation of a wide range of phenomena observed in experiments, as well as identifying the condition for, and transition to, low phase noise operation. We found that the origin of phase noise comes from the non-commensurability of subcombs. Once the subcombs begin to merge, individual resonator modes are populated by multiple lines with slightly different optical frequencies, resulting in multiple and eventually broad beat notes. The critical parameter here is the ratio of the cavity linewidth ? to the dispersion parameter D2, which determines the distance between the primary comb lines and the pump. This work serves as guidelines for the design of Kerr comb generators.

Ultra-stable lasers with high spectral purity are critical tools for many applications in metrology, sensing and spectroscopy. We have demonstrated the frequency stabilization of an external cavity diode laser locked to a whispering gallery mode reference resonator made of single crystal MgF2, and reached a relative Allan deviation of 6×10-14 at an integration time of 100 ms, limited only by the fundamental thermal fluctuation (PRA 2011). The WGM resonator has several advantages over conventional Fabry-Perot (FP) resonators: it is much more compact, requires no mirror coatings, and is wavelength versatile as it can extend from ultraviolet (UV) to mid-infrared (IR). Furthermore, the strong thermal dependence of the difference frequency between two orthogonally polarised TE and TM modes (dual-mode frequency) of the optically anisotropic MgF2 material allows sensitive measurement of the resonator's temperature within the optical mode volume. This dual-mode signal was used as feedback for self-referenced temperature stabilisation to nano kelvin precision, resulting in frequency stability of 0.3 MHz/h at 972 nm (Optics Express, 2012).